The Epidemiology Branch is conducting a number of birth defect studies in collaboration with the Health Research Board and Trinity College, Dublin, Ireland. The main objective of these studies is to determine the relationship between folate and birth defects. The birth defects studied to date are neural tube defects (NTDs), oral clefts, and Down syndrome. These studies focus on biochemical factors in the area of folate metabolism, and on genetic mutations in folate related genes associated with birth defects. In the past we have shown that elevated homocysteine is a risk factor for NTDs, that a mutation in the methylenetetrahydrofolate reductase (MTHFR) gene 677C->T is a risk factor for both NTDs and oral clefts, and that a small dose of folic acid (100-200 micrograms) can raise red cell folate to levels that can prevent a fifth to almost a half of NTDs. Recently we showed that folic acid supplementation with as little as 100-200 micrograms daily can reduce homocysteine significantly. This is important both for NTD prevention and possible prevention of heart disease and cancer. We have just identified a new gene associated with increased risk for NTDs, this gene is involved in interconversion of tetrahydrofolate. We are currently collecting data and samples for genetic analysis on subjects with oral clefts and their families and samples from children with congenital heart defects and normal controls.. Our genetic studies are currently focused on looking for folate enzyme gene variants that are associated with NTDs.